The present invention relates to an optical pulse generator for use in the fields of optical communications, optical information processing, optical measurement, and spectroscopic or physical property measurements of various materials.
Various methods to generate ultrashort optical pulses by using semiconductor lasers are known, such as the active mode locking, passive mode locking, and gain-switching. The methods of active and passive mode locking are described in detail, for example, in Semiconductors and Semimetals, Vol. 22, Part B, Chapter 1. In the active mode locking, the injection current into the semiconductor laser is modulated at the round-trip frequency of the external cavity. In the passive mode locking, a saturable absorber is contained in the cavity. In both methods, optical pulses are generated at the round-trip frequency of the external cavity, and the pulse width of the optical pulses is generally 1 to 20 ps. In the gain-switching method, a semiconductor laser is excited by a short current pulse, and only the first pulse of the relaxation oscillation caused by the transient response of the laser oscillation is picked up to generate a short pulse of light. In this method, a repetitive frequency of the light pulse may be freely adjustable. On the other hand, the generation of ultrashort optical pulses using a semiconductor laser of the wavelength conversion type is reported by T. Taniuchi, K. Yamamoto and G. Tohmon in "Blue and Ultraviolet Emission from Frequency Doubled Diode Lasers" in Proceedings of 1988 LEOS Annual Meeting (Institute of Electrical and Electronics Engineers, New York, 1988), paper VL 2.4. FIG. 8 shows a schematic structural diagram of a conventional wavelength conversion type ultrashort pulse generator, in which element 1 is a semiconductor laser; element 2 is a wavelength conversion element; element 3 is an optical waveguide; elements 4 and 5 are lenses, and element 6 is a half wave plate. A fundamental wave 7 of TE mode oscillation emitted from the semiconductor laser 1 is collimated by the lens 4, and is converted in the polarization direction by 90 degrees in the half wave plate 6, and is focused by the lens 5 to be fed into the optical waveguide 3 formed in the wavelength conversion element 2. At this time, the phase velocity of the fundamental wave propagating through the optical waveguide 3 and that of the second harmonic wave 8 emitted by Cerenkov radiation are equalized, so that the second harmonic wave is generated efficiently. When the semiconductor laser with the wavelength of 0.84 .mu.m is gain-switched by a short current pulse with the pulse width of several hundred ps obtained as an output of a comb generator, a fundamental wave with pulse width of about 20 ps is generated, so that a second harmonic wave of wavelength of 0.42 .mu.m with pulse width of about 10 ps is obtained.
In the active or passive mode locking method of semiconductor laser, since the light output of the semiconductor laser is fed back again to the semiconductor laser through the external cavity, the output cannot be increased sufficiently. Besides, the construction of the apparatus is complicated, and the adjustment is difficult. On the other hand, in the gain-switching method of semiconductor laser, ultrashort light pulses may be generated in a simple construction. However, when the output is subjected to wavelength conversion, it is difficult to heighten the output of the radiated light pulse of the semiconductor laser since the output of the second harmonic wave obtained is small. The output light pulse of the semiconductor laser may be increased by elevating the peak value of the exciting current pulse, but if the pulse width of the current pulse is very small, on the order of several hundreds ps, it is difficult to obtain current pulses of a high peak value.